1. Field of the Invention
The present invention relates to a monolithic LC filter to be used in mobile communication devices such as cellular telephones.
2. Related Art
Various conventional miniaturized, small-loss monolithic LC filters suitable for mass production have been proposed and have been used in mobile communication devices such as cellular telephones. Such a monolithic LC filter typically includes a first LC resonance circuit including a first resonator inductor, and a second LC resonance circuit including a second resonator inductor. A conventional monolithic LC filter is shown in FIG. 5, wherein a monolithic LC filter Axe2x80x2 is configured in such a manner that a plurality of electrodes or conductor patterns t of various shapes are laminated together with dielectric layers S interposed therebetween. More specifically, the monolithic LC filter Axe2x80x2 of FIG. 5 has a rectangular parallelepiped shape that is fabricated through a process in which a conductor pattern or conductor patterns (corresponding to electrodes t) are formed on each of a plurality of rectangular dielectric sheets (corresponding to dielectric layers 5), the plurality of dielectric sheets are laminated, and the resultant laminated body is fired.
Considering the inductors in more detail, the first and second resonator inductors are formed of generally C-shaped lines or conductive elements indicated at 5 and 6, respectively, which are formed on a common dielectric layer (inductor layer L) so that portions thereof extend in parallel. More specifically, as shown in FIG. 6, line 5 includes a straight coupling section 5a for establishing a magnetic field coupling, lead sections 5b which extend outwardly from opposite ends of the coupling section 5a, and connection sections 5c, with a first end of each of the connection sections 5c being connected to the corresponding lead section 5b and a second end thereof being connected to a conductor pattern of another dielectric layer. Similarly, line 6 includes a straight coupling section 6a for establishing magnetic field coupling, lead sections 6b which extend outwardly from opposite ends of the coupling section 6a, and connection sections 6c, with a first end of each of the connection sections 6c being connected to the corresponding lead section 6b and a second end thereof being connected to a conductor pattern of another dielectric layer. The coupling section 5a is formed so as to extend parallel to a virtual or imaginary line k passing axially through the connection sections 5c, and the coupling section 6a is formed so as to extend parallel to a virtual or imaginary line k passing axially through the connection sections 6c. 
When strong magnetic field coupling is required for a monolithic LC filter having the aforementioned configuration, the lengths of the coupling sections which establish the magnetic field coupling must be increased. However, because there is a strong demand in this field for components of reduced size, a limitation is imposed on the lengths of sides of the dielectric layers on which the coupling sections are provided. Therefore, a problem is encountered in attempting to increase the lengths of the coupling sections, with the result that the requirement for strong magnetic field coupling cannot be fully satisfied with the configuration shown without increasing the size of the corresponding dielectric layer.
In view of the foregoing, an object of the present invention is to provide a monolithic LC filter which can establish magnetic field coupling of an increased strength as compared with a conventional monolithic LC filter, without a corresponding increase in component size.
In accordance with the present invention, there is provided a monolithic LC filter comprising: a first LC resonance circuit including a first resonator inductor; and a second LC resonance circuit including a second resonator inductor, the first and second resonator inductors comprising conductive lines or elements formed on a common dielectric layer, said lines including respective coupling sections that extend in parallel with each other so as to establish magnetic field coupling between the first and second resonator inductors, each of the lines further including, at opposite ends thereof, a connection section connected to a conductor pattern of a further dielectric layer of the filter, the coupling section of each of the lines being formed so as to be inclined with respect to a virtual line passing axially through the corresponding connection sections.
By virtue of this configuration, the lengths of the coupling sections which establish magnetic field coupling are greater than the length of the coupling sections of a conventional monolithic LC filter (such as that shown in FIGS. 5 and 6), and thus the magnetic field coupling can be strengthened without an attendant increase in the size of the LC filter.
Preferably, the monolithic LC filter is of a rectangular parallelepiped shape, the dielectric layer has a rectangular shape, and the coupling sections are formed on the dielectric layer so as to extend generally along a diagonal of the dielectric layer. This configuration enables the lengths of the coupling sections to be increased to a maximum value in a dielectric layer having a limited surface area.
Preferably, each of the lines has a lead section which extends between one of the connection sections and the corresponding end of the coupling section along an edge of the dielectric layer. This configuration enables the lines to be connected to a conductor pattern of another dielectric layer of the filter while providing efficient utilization of a dielectric layer having a limited surface area.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.